In processes of the above type for the production of nitric acid, it is necessary to prevent or at least suppress undesirable side reactions of the reactants or to remove unwanted by-products in a manner that causes the least disturbances and to minimize material and energy requirements or to compensate them against each other.
For the production of nitric acid from reaction gas originating from an ammonia combustion unit, plants of the so-called "dual pressure" type are known, in which the combustion takes place at a pressure of about Pabs=2-7 bar and the absorption at about Pabs=8-20 bar. Such dual pressure plants are characterized by a high NH.sub.3 yield, low platinum losses and long operating periods of the combustion unit on the one hand, and by a high degree of oxidation and low reaction volume in the absorption unit on the other hand. However, dual pressure plants require a gas compressor between the combustion and absorption units for raising the pressure of the combustion gases. The hot reaction gas from the combustion unit is cooled by known means in heat exchangers for steam generation, tail gas preheating and in water coolers to about 50.degree. C. before being fed to the suction side of the gas compressor where the pressure is raised to the particular operating pressure selected for the downstream absorption unit. Although such dual pressure plants frequently represent the best solution from the economic point of view, certain risks are involved as regards the reliability and safety of the plant. Thus, chemical side reactions between unreacted ammonia from the ammonia combustion unit with nitrogen oxides result in the formation of ammonium salts in the reaction gas. The ammonium salts tend to settle in the suction nozzle of the gas compressor and in the compressor itself, one of the results of this being higher driving power requirements. Even if this fact can be tolerated as an unavoidable phenomenon, these deposits also constitute an increased safety risk due to the explosion hazard.
In the event of an explosive decomposition of the ammonium nitrate, which is quite likely to occur, and in case of ammonium nitrate deposits which could happen during the start-up phase, this risk is considerably greater, since not only can the compressor and other equipment be damaged beyond repair, but personnel may also suffer injuries.
As no means could be found to prevent such deposits, measures have to be taken for their removal. The gas compressor is equipped with facilities for injecting water continuously or at intervals. Although the deposited matter is easily soluble in water, it is still impossible to remove all the deposits because the complicated geometry of the compressor casing prevents the water from reaching all parts of the compressor. Moreover, the injection of water poses certain problems, such as erosion in the gas ducts, sudden changes of the casing temperature, and the premature formation of nitric acid which will evaporate on entering the subsequent compressor stages thus causing corrosion.